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With the availability of A-ring labelled 25OHD2, [3alpha-3H] 25OHD2, we have performed the present study to examine the metabolism of 25OHD2 using physiological substrate concentrations in perfused kidneys isolated from both normal and vitamin D2-intoxicated rats. Our results indicate that [3alpha-3H] 25OHD2 is metabolized into both 24(S),25,28-trihydroxyvitamin D2 [24(S),25,28(OH)3D2] and 24(R),25,26-trihydroxyvitamin D2 [24(R), 25,26(OH)3D2], and the amounts of these two metabolites produced in the kidney of vitamin D2-intoxicated rat were about 3-5 times higher than those produced in the kidney of normal rat. Similar results were also obtained with rat kidney homogenates incubated with [3alpha-3H] 25OHD2. Furthermore, we noted that the production of both 24(S),25,28(OH)3D2 and 24(R),25,26(OH)3D2 in the kidney homogenates of vitamin D2-intoxicated rats increased with the time of incubation and then subsequently decreased. The decrease in both 24(S),25,28(OH)3D2 and 24(R),25,26(OH)3D2 coincided with an increase in the fraction of total radioactivity distributed in the aqueous phase of the kidney homogenates. This finding suggested the possibility of further metabolism of 24(S),25,28(OH)3D2 and 24(R), 25,26(OH)3D2 into polar water-soluble metabolite(s). We then measured the radioactivity in the aqueous phase of kidney homogenates of both normal and vitamin D2-intoxicated rats incubated with [3alpha-3H] 25OHD2. It was noted that the amount of radioactivity in the aqueous phase of kidney homogenates of vitamin D2-intoxicated rats is higher than that present in the aqueous phase of kidney homogenates of normal rats. Thus, our study provides evidence for the first time for the formation of both 24(S),25,28(OH)3D2 and 24(R),25, 26(OH)3D2 under physiological conditions, and the possibility of their further metabolism into as yet unidentified polar water-soluble metabolite(s). As the formation of all these metabolites is increased in the kidney of vitamin D2-intoxicated rats when compared to normal rats, it appears that the increased rate of metabolism of 25OHD2 during hypervitaminosis D2 plays a significant role in the deactivation of 25OHD2.

Metabolism of [3.ALPHA.-3H] 25-Hydroxyvitamin D2 in Kidneys Isolated from Normal and Vitamin D2-intoxicated Rats.